The present invention relates to a process and to an apparatus for the polymerization and/or crosslinking of a resin used in the composition of a composite material part using ionizing radiation. It makes it possible on an industrial scale to polymerize and/or crosslink resins used in the composition of thick or solid parts, i.e. parts having a surface mass which can locally or entirely exceed 4 g/cm.sup.2.
These composite material parts are of two types, namely parts having a simple structure and those having a complex structure. The former are formed from elements such as fibres, grains, powders, etc bonded to one another by hardening resins, said elements being regularly distributed in space. The latter are formed in the same way as the former, but also have e.g. metallic or radio hardening objects or inserts, such as of rubber, which are entirely or partly incorporated into the resin. These parts having a simple or complex structure are more particularly used in the motor vehicle, aeronautical, space and similar fields.
The hardening processes leading to the polymerization and/or crosslinking of the resins include a first group of so-called conventional processes covering on the one hand those using cold resin systems, i.e. polymerization in the open air without any heat supply, and on the other hand processes using a resin system, whereof the polymerization is obtained by raising the temperature in the oven. These processes are respectively called cold and hot conventional processes.
The cold conventional processes are difficult to perform and give materials with a very poor thermal behavior. The hot conventional processes give better materials, but suffer from faults due to thermal effects, such as internal stresses, cracks, delaminations in the case of thick parts, whilst the polymerization times lead to long production cycles having serious financial consequences from the investment and energy consumption standpoints.
A second group of processes called "by radiation" also include processes without raising the temperature and processes involving raising the temperature. The latter more particularly include microwaves, high frequency, infrared and suffer from the disadvantages of the so-called hot conventional processes.
Processes by radiation such as .lambda., .gamma. and UV as well as electrons permit a polymerization and/or crosslinking without raising the temperature, these consisting of ionizing rays.
Relatively low energy ultraviolet rays permit a slight penetration of composite materials. Gamma rays suffer from the disadvantage of not being focused, which leads to a very slow polymerization and/of crosslinking. In addition, it is difficult to use these rays because they come from permanent radioactive sources, which are consequently difficult to industrially transpose.
X-rays, obtained from existing X-ray generators are not very suitable for the polymerization and/or crosslinking of thick parts due to lack of power, so that there is a low dose rate and inadequate dimensioning of the beam.
Electron bombardment or radiation is a very efficient, high performance process. In particular, it makes it possible to obtain very high quality composite materials in a relatively short time and using relatively low energy levels. Moreover, it is compatible with all fibre types used in the composition of composite materials.
However, the penetration of the electrons into the composite material is dependent on the characteristics of the energy generator, the density of the material and to a lesser extent the absorption coefficient thereof. The penetration of the parts by electron radiation is 0.35 to 0.45 g/cm.sup.2 /MeV and is consequently equivalent to the penetration into a material having a surface mass varying from 3.5 to 4.5 g/cm.sup.2 for a generator with an energy level of 10 MeV. The surface mass value is dependent on variations of the absorption coefficients of the materials forming the structure.
Apart from the disadvantages of processes by radiation accompanied by a temperature rise, the use of infrared only permits a limited penetration of the composite material and which is at the most equal to 1 millimeter, penetration being limited by certain fibers, such as carbon in the case of the microwave process (UHF).
With regards to UHF, the use of such rays is still at the research stage and consequently industrial development is virtually non-existent, particularly for the production of parts having a complex structure. Moreover, the use thereof for complex parts leads to significant realization problems and they are generally only usable for parts having a simple shape, such as parallelepipedic or cylindrical.